1. Field of the Invention
The present invention relates to a broadcast receiver for receiving airwaves of a predetermined channel sent from plural broadcast stations by a directional switching antenna.
2. Description of the Related Art
In Japan, airwaves of television broadcasts etc. are generally sent from a base station of one place. As a result of that, almost all the airwaves sent by plural broadcast stations can be received by directing an antenna for broadcast receiving to the base station and fixing the antenna to a roof or a veranda, etc. of a house and connecting the antenna to a broadcast receiver through a cable. On the other hand, for example, in the U.S.A., airwaves are individually sent from plural broadcast stations, respectively. As a result of that, when an antenna is directed at one place and is fixed, airwaves of the broadcast station located in a direction in which the antenna is directed can be received, but airwaves of the broadcast station located in a direction in which the antenna is not directed cannot be received.
Therefore, it is necessary to switch directivity of an antenna in plural directions in order to receive radio waves from plural broadcast stations located in different bearings by one antenna. An antenna capable of switching such directivity (hereinafter called “a directional switching antenna”) is described in, for example, JP-A-2001-160773 (Paragraphs 0014 to 0017, FIG. 1), JP-A-5-232205 (Paragraphs 0009 to 0018, FIG. 4) and JP-A-56-17536 (Page 5, Line 5 of the upper left field to Page 5, Line 12 of the lower right field), mentioned below. The directional switching antenna is constructed of plural antenna elements and phase shifters, etc. corresponding to each of the antenna elements, and directivity is electrically varied by adjusting phases of signals received in each of the antenna elements by each of the phase shifters and combining the phases. For example, in a smart antenna which is one example of the directional switching antenna, it is stipulated that directivity should be switched in 16 directions.
In the case of using such a directional switching antenna, airwaves from each of the broadcast stations can be received by sending a control signal from a broadcast receiver to the antenna and switching directivity of the antenna in a predetermined direction. As a result of this, a user can receive almost all the airwaves sent from plural broadcast stations interspersed even when the directional switching antenna is fixed and attached to a roof etc. of a house.
Control methods of the directional switching antenna are shown in JP-A-2001-160773, JP-A-5-232205 and JP-A-56-17536. In JP-A-2001-160773, it is constructed so that a load on a phase shifter is reduced to decrease a failure rate by performing control in which frequency of a beam oriented direction in which a received signal becomes a maximum level is counted and the beam oriented direction is fixed in a direction with the largest frequency to conduct communication and when the communication is blocked, the beam oriented direction is switched in a direction with the second largest frequency to conduct communication and when the communication is blocked, the beam oriented direction is switched in a direction with the third largest frequency. In JP-A-5-232205, it is constructed so that when a receiving level in the present oriented direction becomes lower than a reference value, the oriented direction is respectively changed by one step clockwise and counterclockwise and when a receiving level in the oriented direction after the change is higher than the receiving level in the present oriented direction, its direction is set as a new oriented direction and thereby a satellite can be tracked with high accuracy. In JP-A-56-17536, it is constructed so that in the case of switching an oriented direction, directivity is periodically changed by a small amount and a switching direction is decided based on a change state of a received signal at that time and thereby a received signal level is prevented from dropping in a transition process of the oriented direction and stable reception can be performed.
The broadcast receiver for receiving airwaves by the directional switching antenna as described above is provided with a function of automatic scanning so that at the time of selecting a channel, a direction in which airwaves of its channel can be received with the highest sensitivity is retrieved and directivity of the antenna can be adjusted to its direction. In this automatic scanning, with respect to each of the channels, the directivity of the antenna is sequentially switched and a level of a received signal in each of the directions is detected. Then, when the level of the received signal is a threshold value or more, it is determined that its direction is a receivable direction in the corresponding channel, and further a direction with the largest reception level (that is, high reception sensitivity) among the receivable directions is decided as an optimum direction in its channel. Then, airwaves of its channel can be received in the best state by electrically controlling the antenna so that the directivity of the antenna becomes the optimum direction.
FIG. 12 is a diagram describing a method of automatic scanning at the time of channel selection. Here, the case of a smart antenna capable of switching directivity in 16 directions is taken as an example. Numeric characters 0 to 15 in the drawing indicate numbers of the directions. As shown in FIG. 12, when one channel is selected, for example, scanning is started from the direction 0 and while scanning is sequentially performed counterclockwise, levels of received signals in the respective directions are detected and it is determined whether or not the levels are a threshold value or more. This action is called seek processing. This processing is performed with respect to all the directions ranging from the direction 0 to the direction 15 and capability or incapability of reception is determined every each of the directions. Here, the directions 9, 10, 11 and 12 are in states capable of reception. Then, at a point in time when the scanning is performed to the direction 15 and the seek processing is ended, the direction with the largest reception level among the receivable directions, that is, the direction with the highest reception sensitivity is decided as an optimum direction in its channel. Here, the direction 11 is in a state of the optimum direction.
By the way, in a broadcast receiver, a preset function for setting channels capable of being viewed in a use region of the receiving apparatus is required like a general television image receiving apparatus. Then, in order to automatically make this presetting, a function of automatic scanning for sequentially switching directivity of an antenna and retrieving receivable channels is required.
However, when the same algorithm as that of the automatic scanning at the time of normal channel selection as described above is used as algorithm of the automatic scanning in the case of presetting a channel, seek processing is performed over all the directions of the antenna every each of the channels and processing for detecting the optimum direction from among receivable directions is performed, so that there is a problem that it takes time to complete the automatic scanning for all the channels and waiting time of a user becomes long.
The automatic scanning at the time of presetting a channel is originally processing performed to retrieve receivable channels, and seek processing for performing detection to the optimum direction is not required in the case of only retrieving the receivable channels. The invention has been implemented from such a standpoint, and an object of the invention is to achieve a speedup in automatic scanning at the time of presetting a channel in a broadcast receiver for receiving airwaves by a directional switching antenna.